Schrödinger's cat is referenced in "The Tangerine Factor" when both Penny and Leonard seek the "wisdom” of reluctant adviser, Sheldon, on separate occasions. Schrödinger's Cat is posited by Sheldon to serve as an analogue to Leonard and Penny's potential relationship prior to their first date. Penny is told that her date could be considered to be a superposition of two mutually contradictory states (good and bad) until the box is opened, giving her incentive to proceed. He gives the same advice to Leonard. When Penny and Leonard meet for their date, they kiss with Penny remarking that "The cat's alive!" In "The Codpiece Topology", she is shown explaining Schrödinger's Cat to another date (Eric) as they ascend the stairs leading to her apartment. In "The Russian Rocket Reaction", Sheldon invokes this application of Heisenberg's uncertainty principle to convey his dissatisfaction with Leonard's perceived betrayal in deciding to attend a party hosted by his archenemy, Wil Wheaton. For a time, Sheldon considered Leonard to be simultaneously his friend and not his friend and characterized this phenomenon as 'Schrödinger's Friendship' aldso in "The Russian Rocket Reaction".
In physics, Schrödinger's Cat is a Thought Experiment or Paradox devised in 1935 by Austrian physicist Erwin Schrödinger. Illustrating the Copenhagen interpretation of Quantum mechanics as applied to everyday objects, the experiment describes a cat that might be alive or dead in a sealed box and is often featured in theoretical discussions of the interpretation of quantum mechanics.
In Die gegenwärtige Situation in der Quantenmechanik, Schrödinger decribes it as: "One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of Radioactive substance, so small that perhaps in the course of the hour, one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges, and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts. It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks."